1. Field of the Invention
The present application relates to an amplifier circuit, in particular, the application relates to a type of, what is called, the current reuse amplifier.
2. Related Background Art
A current reuse amplifier has been well known in the field, where a current supplied to the downstream stage is commonly guided to the upstream stage. A Japanese Patent Application published as JP-2008-035083A has disclosed one type of the current reuse amplifier. One of advantages of the current reuse amplifier is that two or more transistors in respective stages are connected in series between a power supply and the ground, which effectively suppresses the current consumption of the amplifier.
In a low noise amplifier (LNA) and/or a power amplifier with the multi-stage arrangement, the first stage transistor preferably has a smaller size compared to transistors in the second or the downstream stages in order to enhance the noise figure (NF), while, transistors in the downstream stages preferably has a larger size to enhance the linearity thereof. In the current reuse amplifier, however, because the DC current commonly flows in transistors in the first stage and the second or downstream stages, the transistors are necessary to have the size same to each other. When the second or the downstream transistors have the size same to that of the first stage, the downstream transistors easily saturate and become unable to show enough linearity. On the other hand, when the first transistor has the size same with those of the downstream transistors, the first transistor shows a degraded NF because of inadequate biases.